It may not be your solution, but...

I recently fixed a customer's laptop which showed the same symptoms. It started happening after AVG found and removed the file antispyware.srv.exe. It had installed itself as a service that would start even in Safe Mode, and if it failed to load, it's first recovery action was to reboot the computer. I removed the service by deleting the registry key HKLM\System\CurrentControlSet\Services\Antispyware .

I would suggest booting into "Last Known Good", check the Application and System event logs for services that failed to load (mine was "Antispyware Scanning Engine" in the System log), run services.msc, and look at the recovery tab of the service(s) to see it's actions when it fails.

Hope this helps.

eDanix